1. Field of the Invention
This invention relates to hydraulically setting materials reinforced with polyvinyl alcohol synthetic fibers (hereinafter referred to as PVA fibers).
2. Description of the Prior Art
As has hitherto been well known, asbestos fibers are representative inorganic fibers for forming hydraulically setting materials such as cement etc. and further exerting a reinforcing effect thereon. However, the supply of these asbestos fibers depends on the imports and the number of the producing countries is greatly limited from a global aspect, and also it is expected that their sources will be depleted and the asbestos fibers will inevitably be less available. Also from the standpoint of harmness to human health, there is danger concerning the environmental hygiene and pollution, for example, it is advisable to avoid inhalation of asbestos fibers, and therefore it is now under study to replace these asbestos fibers.
The present inventors have been studying fiber-reinforced hydraulically setting materials employing PVA fibers as substitute fibers for asbestos fibers and have come to the present invention, and therefore the present invention aims to present hydraulically setting materials having excellent folding endurance comparable to or better than the asbestos fiber-reinforced materials by employing PVA fibers.
The characteristics required of a fiber for reinforcing cement include, in addition to the fundamental mechanical properties such as strength, modulus etc., the shape factors of the fiber, i.e. the fiber diameter and fiber length, further properties relating to the stability in a cement material constituting a matrix, still further properties represented by the surface performance of the fiber, for example, interfacial bond strength, and the like, but apart from these required characteristics, it is a primary requisite for manifesting a reinforcing effect to actually disperse the reinforcing fiber sufficiently in the cement, and if the dispersion is inadequate, the reinforcing effect is greatly reduced. In general, dispersibility has a tendency to decrease with the reduction in thickness of the fiber. In fact, for example with glass fibers etc., single fibers are deliberately collected as chapped strands to make them thicker in order to improve such properties. Furthermore, also with synthetic fibers, in order to avoid aggravation of the dispersion, only those of several or higher denier are employed, and there is no literature relating to the region where the fineness is small.
In other words, the present situation is such that depending on the balance of the dispersibility and the reinforcing factors relating to the fiber shape, the reinforcing effect cannot be manifested unless a specific range of the fiber diameter is selected. Also with the case where PVA fibers are employed, known literature is concerned only with the use of 2 denier or higher in fear of poor dispersion, and no investigation has been made on the use of fibers of a smaller diameter range. In short, it is a matter of common knowledge that with less than 2 denier, the dispersion is poor and hence the reinforcing effect is small.
However, the present inventors have paid attention to that the PVA fibers are hydrophilic in nature and their dispersibility is relatively good as compared with other hydrophobic synthetic fibers or inorganic fibers and, based on this, have enabled adequate dispersion even with less than 2 denier (dr) by selecting the aspect ratio of the PVA fiber to be used and the dispersing agent for enhancing the dispersibility, thereby having been successful in obtaining a remarkable reinforcing effect.